From patchwork Wed Jan 6 17:40:28 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Vlastimil Babka X-Patchwork-Id: 12001989 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-16.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 63ABCC433E0 for ; Wed, 6 Jan 2021 17:40:47 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id DC4022312C for ; Wed, 6 Jan 2021 17:40:46 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org DC4022312C Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.cz Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 161806B02B4; Wed, 6 Jan 2021 12:40:46 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 0F65E6B02B5; Wed, 6 Jan 2021 12:40:46 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 025B16B02B5; Wed, 6 Jan 2021 12:40:45 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0081.hostedemail.com [216.40.44.81]) by kanga.kvack.org (Postfix) with ESMTP id DFDD56B02B3 for ; Wed, 6 Jan 2021 12:40:45 -0500 (EST) Received: from smtpin01.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id A2F92824556B for ; Wed, 6 Jan 2021 17:40:45 +0000 (UTC) X-FDA: 77676065250.01.mine40_2e083d0274e3 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin01.hostedemail.com (Postfix) with ESMTP id 7BBD01004C782 for ; Wed, 6 Jan 2021 17:40:45 +0000 (UTC) X-HE-Tag: mine40_2e083d0274e3 X-Filterd-Recvd-Size: 4247 Received: from mx2.suse.de (mx2.suse.de [195.135.220.15]) by imf18.hostedemail.com (Postfix) with ESMTP for ; Wed, 6 Jan 2021 17:40:44 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 93798AD7A; Wed, 6 Jan 2021 17:40:43 +0000 (UTC) From: Vlastimil Babka To: linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Vladimir Davydov , Qian Cai , David Hildenbrand , Michal Hocko , Vlastimil Babka Subject: [RFC 2/3] mm, slab, slub: stop taking cpu hotplug lock Date: Wed, 6 Jan 2021 18:40:28 +0100 Message-Id: <20210106174029.12654-3-vbabka@suse.cz> X-Mailer: git-send-email 2.29.2 In-Reply-To: <20210106174029.12654-1-vbabka@suse.cz> References: <20210106174029.12654-1-vbabka@suse.cz> MIME-Version: 1.0 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: SLAB has been using get/put_online_cpus() around creating, destroying and shrinking kmem caches since 95402b382901 ("cpu-hotplug: replace per-subsystem mutexes with get_online_cpus()") in 2008, which is supposed to be replacing a private mutex (cache_chain_mutex, called slab_mutex today) with system-wide mechanism, but in case of SLAB it's in fact used in addition to the existing mutex, without explanation why. SLUB appears to have avoided the cpu hotplug lock initially, but gained it due to common code unification, such as 20cea9683ecc ("mm, sl[aou]b: Move kmem_cache_create mutex handling to common code"). Regardless of the history, checking if the hotplug lock is actually needed today suggests that it's not, and therefore it's better to avoid this system-wide lock and the ordering this imposes wrt other locks (such as slab_mutex). Specifically, in SLAB we have for_each_online_cpu() in do_tune_cpucache() protected by slab_mutex, and cpu hotplug callbacks that also take the slab_mutex, which is also taken by the common slab function that currently also take the hotplug lock. Thus the slab_mutex protection should be sufficient. Also per-cpu array caches are allocated for each possible cpu, so not affected by their online/offline state. In SLUB we have for_each_online_cpu() in functions that show statistics and are already unprotected today, as racing with hotplug is not harmful. Otherwise SLUB relies on percpu allocator. The slub_cpu_dead() hotplug callback takes the slab_mutex. To sum up, this patch removes get/put_online_cpus() calls from slab as it should be safe without further adjustments. Signed-off-by: Vlastimil Babka --- mm/slab_common.c | 10 ---------- 1 file changed, 10 deletions(-) diff --git a/mm/slab_common.c b/mm/slab_common.c index e728265c8b7d..0f29a2b59dac 100644 --- a/mm/slab_common.c +++ b/mm/slab_common.c @@ -310,8 +310,6 @@ kmem_cache_create_usercopy(const char *name, const char *cache_name; int err; - get_online_cpus(); - mutex_lock(&slab_mutex); err = kmem_cache_sanity_check(name, size); @@ -360,8 +358,6 @@ kmem_cache_create_usercopy(const char *name, out_unlock: mutex_unlock(&slab_mutex); - put_online_cpus(); - if (err) { if (flags & SLAB_PANIC) panic("kmem_cache_create: Failed to create slab '%s'. Error %d\n", @@ -487,8 +483,6 @@ void kmem_cache_destroy(struct kmem_cache *s) if (unlikely(!s)) return; - get_online_cpus(); - mutex_lock(&slab_mutex); s->refcount--; @@ -503,8 +497,6 @@ void kmem_cache_destroy(struct kmem_cache *s) } out_unlock: mutex_unlock(&slab_mutex); - - put_online_cpus(); } EXPORT_SYMBOL(kmem_cache_destroy); @@ -521,14 +513,12 @@ int kmem_cache_shrink(struct kmem_cache *cachep) { int ret; - get_online_cpus(); mutex_lock(&slab_mutex); kasan_cache_shrink(cachep); ret = __kmem_cache_shrink(cachep); mutex_unlock(&slab_mutex); - put_online_cpus(); return ret; } EXPORT_SYMBOL(kmem_cache_shrink);